The present invention refers to a method of and apparatus for detecting the presence of vapor and/or smoke in the outgoing air of a device for heating materials, especially of a microwave unit for disinfection and sterilization of goods.
A heating of materials, in particular for disinfecting or sterilizing goods, requires a monitoring of the temperature and of the state of aggregation of the material, not only to prevent a drying out and possibly an ignition of the material but also to unequivocally determine the beginning of the actual treatment process such as boiling.
Known smoke detectors operate in various ways, for example by measuring the intensity decrease of a direct optic path. Such devices are, however, incapable of differentiating smoke particles from vapor particles because the receiver detects the radiant intensity irrespective whether the radiant intensity has been diminished by the smoke particles or by the vapor particles.
In particular in connection with appliances such as microwave units for disinfecting and sterilizing goods, it is necessary to register the temperature increase of the goods placed in the radiation chamber in order to avoid a heating above the point of ignition or to recognize an occurring smoldering fire at the earliest time. Also, one should be able to clearly determine the start of the treatment period. Direct measurement of the temperature of the goods to be treated by means of expensive measuring probes becomes even more expensive and complicated in view of the prevailing high frequency field. Thus, hygienic and practical reasons prohibit also a routine monitoring. Thence, a simple measurement of the temperature increase of the goods to be treated can be performed only through indirect means.
It was therefore proposed to install a humidity sensor within the exhaust system of a microwave unit for measuring and registering the humidity content of the outgoing air. Even though this may allow two different measurements, that is on the one hand, the measurement of the intensity decrease of a direct optic radiation and, on the other hand, the indirect measurement of the humidity by means of the humidity sensor within the exhaust system; the use of a humidity sensor is still disadvantageous because the humidity sensor is subjected to wear and easily fails under certain operational conditions so that expensive and cumbersome repair work becomes necessary.
Also, conventional smoke detectors provided within the exhaust system and operating with infrared have a tendency for easy breakdown in view of external conditions, especially when aggressive gases or vapors are contained in the outgoing air of the material which impair or even may destroy the gages. Further, thc use of piezoelectric instruments were proposed to measure the degree of moisture of the outgoing air. However, their use is also not possible when being subjected to aggressive mediums.
In general, it should be noted that only those devices are applicable for microwave units which are not influenced or destroyed by a high-frequency field. Apart from conventional detectors having drawbacks as previously set forth, none of the conventional devices are stable within the high-frequency field.